1. Field of the Invention
This invention relates generally to the field of integrated circuit design and, more particularly, to the design of temperature measurement devices.
2. Description of the Related Art
Many digital systems, especially those that include high-performance, high-speed circuits, are prone to operational variances due to temperature effects. Devices that monitor temperature and voltage are often included as part of such systems in order to maintain the integrity of the system components. Personal computers (PC), signal processors and high-speed graphics adapters, among others, typically benefit from such temperature monitoring circuits. For example, a central processor unit (CPU) that typically “runs hot” as its operating temperature reaches high levels may require a temperature sensor in the PC to insure that it doesn't malfunction or break due to thermal problems.
Often, integrated circuit (IC) solutions designed to measure temperature in a system will monitor the voltage across one or more PN-junctions, for example a diode or multiple diodes at different current densities to extract a temperature value. This method generally involves amplifying (or gaining up) a small voltage generated on the diode(s), and then subtracting voltage from the amplified temperature-dependent voltage in order to center the amplified (gained) value for conversion by an analog-to-digital converter (ADC). In other words, temperature-to-digital conversion for IC-based temperature measuring solutions is often accomplished by measuring a difference in voltage across the terminals of typically identical diodes when different current densities are forced through the PN junctions of the diodes. The resulting change (ΔVBE) in the base-emitter voltage (VBE) between the diodes is generally proportional to temperature. (It should be noted that while VBE generally refers to a voltage across the base-emitter junction of a diode-connected transistor and not a voltage across a simple PN-junction diode, for the sake of simplicity, VBE is used herein to refer to the voltage developed across a PN-junction in general.) More specifically, a relationship between VBE and temperature is defined by the equation
                              V          BE                =                  η          ⁢                      kT            q                    ⁢          ln          ⁢                                          ⁢                      I                          I              S                                                          (        1        )            where η is the ideality factor of the PN junction, k is Boltzman's constant, q is the charge of a single electron, T represents absolute temperature, Is represents saturation current and I represents the collector current. A more efficient and precise method of obtaining ΔVBE is to supply the PN junction of a single diode with two separate and different currents in a predetermined ratio. Consequently, ΔVBE may be related to temperature by the equation
                              Δ          ⁢                                          ⁢                      V            BE                          =                  η          ⁢                                          ⁢                      kT            q                    ⁢                      ln            ⁡                          (              N              )                                                          (        2        )            where N is a constant representing a preselected ratio of the two separate currents that are supplied to the PN junction of the diode.
A typical dynamic range of ΔVBE, however, is small relative to dynamic ranges that are typical of analog-to-digital converters (ADCs). That is, ΔVBE, which is used to measure the PN junction temperature, generally has a small dynamic range, for example on the order of around 60 mV in some systems. Therefore it is generally required to further process ΔVBE in order to match the dynamic range of ADCs. Typically, in order to obtain the desired conversion values at various temperatures, ΔVBE is multiplied by a large gain, and then centered to zero, which can be accomplished by subtracting a fixed voltage.
In general, implementations today perform the temperature signal processing (TSP) in a separate temperature sensor circuit that generates a sufficiently large voltage signal, which is fed into a separate ADC that may have been designed using a number of different topologies. Temperature-to-digital converters (TDC) of such implementations usually contain complex circuits with high power dissipation. The yield of these TDCs during the fabrication process may also be low as there are many components that need to be matched for a given process spread.
An example of a typical temperature measurement system, which includes an ADC, is illustrated in FIG. 1. A TSP circuit 100 is coupled to an ADC 130. TSP 100 may comprise current sources 104 and 106, where a current provided by 104 is an integer (N) multiple of a current provided by 106, a diode 102, an integration capacitor 126, an offset capacitor 122, a gain capacitor 124, and an operational amplifier (OP-AMP) 120, interconnected as illustrated in FIG. 1. P1 110 and P2 112 represent non-overlapping clocks that provide switching between two circuit configurations as shown. When P1 110 is closed, current source 104 powers TSP 100 and P2 112 is open. Similarly, when P2 112 is closed, current source 106 powers TSP 100 and P1 110 is open. Switching between current sources 104 and 106, different currents are forced through the junction of diode 102 resulting in a change in diode-junction-voltage (ΔVBE). Although omitted in FIG. 1, it should be understood that when either P1 110 or P2 112 is open, the respective uncoupled current source may be shunted to ground. In the circuit configuration shown, voltage sampling occurs when P1 110 is closed, and charge transfer takes place when P2 112 is closed. In other words, during operation, switching from a configuration of P1 110 closed and P2 112 open to a configuration of P1 110 open and P2 112 closed, results in ΔVBE effectively “pumping” charge to gain capacitor 124, which in turn leads to integration capacitor 126 also receiving a charge. More specifically, opening P1 110 and closing P2 112 results in a value drop of diode-junction-voltage VBE, expressed as ΔVBE. Consequently, ΔVBE appears across the terminals of capacitor 126, in case capacitor 126 is equal in value to capacitor 124. If capacitor 124 is greater in value than capacitor 126, then ΔVBE will be amplified, or “gained up”, hence an amplified value Vtemp 131 will appear at the output of OP-AMP 120. Voffset 132 is subtracted through offset capacitor 122.
Voltage-temperature relationships characterizing TSP 100 may be described by the following equations:Vtemp=CT/CI*ΔVBE(T)−CO/CI*Voffset, whereCT/CI=(ADC dynamic range)/(ΔVBE(Tmax)−ΔVBE(Tmin)), andVoffset=(CT/CI*ΔVBE(Tmax)−(ADC dynamic range))*CI/CO.Tmax and Tmin represent maximum and minimum diode temperatures, respectively. ADC dynamic range indicates a range of valid voltage values required for proper ADC operation.
One disadvantage of temperature measurement systems employing diodes—or generally PN-junctions—is that parasitic resistance in series with the diode(s) typically causes errors in the temperature measurement. Such parasitic resistance may be trace resistance or bulk resistance of the semiconductor material of the diode(s). Typically, in order to substantially eliminate series resistance errors during temperature measurements, multiple current sources are used and the VBE across the diode (PN-junction) is calculated for each current source being individually applied to the diode. Subsequently, the corresponding series resistance is calculated and the error resulting from the series resistance is subtracted from the temperature measurement to cancel the error out.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.